In X-ray sources, for example in X-ray tubes, electrons with kilovolt energies impact onto the focal track of an anode, for example a rotating anode, for generating X-ray radiation. This leads among others to modifications of the anode surface. As a consequence, the tube X-ray yield changes over the course of its lifetime, which may have an impact on the X-ray application. In US 2009/0067578 A1, a rotatable anode is described with a structure comprising slits, for example, which structure can be detected if the structure on the anode passes the focal spot. Thus, properties of the focal spot can be determined from changes of the detection signal during operation of the X-ray tube.